The beauty and diversity of a wild reef is well known. Fish and corals have adjusted to their niche.
What about the captive reef? The aquarist does his/her best to meet the needs of the corals (and fish) to try and ensure their health and longevity. The fish and corals have preferences. For a fish example, many wrasse like to bury themselves in sand and so the aquarist provides it in sufficient depth. Corals have a spot on the reef too, where they are subject to varying seawater currents, light intensity and spectrum. The aquarist tries to meet these differing needs as well.
What if the coral receives continuous directional seawater flow, not strong enough to cause damage, will the coral react? Well, yes, it usually will over time. A land based tree subject to nearly continuous directional wind will lean away from the wind, thus reducing the trees resistance to it. So it is with corals – a coral with directional water flow will tend to grow heavily influenced by that flow. If it is a branching coral, probably particularly a ‘hard’ type, it will reduce the resistance to the flow by growing ‘with’ the flow – in other words it will lean away from the flow.
On the wild reef a hard coral near the crest where it is subject to great turbulence and wave power will have short stubby branches to present less resistance to the water. In areas of less flow, the branches will be longer and possibly more slender.
In a similar way some corals react and grow according to light availability. The shapes are natural, and these are the normal shapes the coral is expected to be. The coral in the wave area with short branches receives considerable light as it is near the surface, so the short branches are not a problem. The coral lower down with less seawater turbulence has longer branches and thus more is exposed to the lowered light level. Some corals form horizontal plates which often overlap one another, and these present a large area for light to strike.
I keep a soft coral reef and wonder if the same thing applies to these. I know the shape and the lean of the coral can be influenced by water flow, but wonder if these corals can alter their growth to gather the light they need? What if they need more light and are in a position too low down to get it?
I have been watching the growth of a soft coral for quite a time. It appeared very close to the bottom of the reef, and the growth is slow. The water flow is quite gentle in the coral’s area but is sufficient and multi-directional, and I don’t believe this influences it particularly.
The coral is a normal branching type, that is, it would normally grow vertically with several more or less vertical branches, with a few side growths as well. It started very small, and I assume it is a ‘dropped’ branch or similar as I didn’t place it there.
The aquarium is 24″ in depth and lit by an array of fluorescent tubes (marine white and actinic blue). I assume the coral’s growth is slow as it receives low light.
The interesting thing is, the coral spent a long period not growing toward the light as might have been expected, but growing horizontally. I wonder if this was to gather as much light as possible by presenting the largest surface to the available light as possible? The coral is now showing vertical growth in several places, so maybe it has enough energy now to be able to do so?
I don’t know what the answer really is, but the above seems feasible as fluorescents are not able to punch much light into nearly 24″ of seawater.
There’s a photo to show the coral (it is now about 5″ across) – I’m not a photographer so I apologise for any shortfall.
Aren’t marine aquariums interesting! There’s always something to watch, just for pleasure or to ponder over.